What are your thoughts when manf state that their oil is specifically formulated for their engines?

That could be the case but that does not mean it is superior to top tier competitors.Most of the time it's just marketing hype.

The big question should be, why isn't formulated for their engines? Likely you won't find an real answer out of then. I also would be interested too of who makes that Vanguard 15w50 oil, but I bet that Mobil 1 15w50 is just as good if not better than Vanguard 15w50.